Flat Roof Repair Guide – Easy for DIY or Trade

Flat roof repairs

In this page we will be looking at no surprise, flat roof repair. I will list here some of the most common roofing systems and effective ways of repairing them both temporary, and permanently. We will look at repairs ranging from simple DIY type repair through to more complicated professional quality, longer lasting repairs.

Flat roof repairs tend to fall into three distinct categories for me.

1. Temporary flat roof repair – A short term repair can be easy, quick and cost effective. The purpose is to stop the initial leak from ruining your persian carpet and Barry Manilow albums, whilst buying you some valuable time to think over, save for, or get quotes on your new flat roof, or professional repair.
2. Permanent flat roof repair – Permanent here meaning it will see out the remaining life of the roof covering until it is in such general overall bad condition that a new flat roof is required. Two examples of this would be good quality professional repairs or patches to an otherwise good roof. Or in some instances a ‘layer on top’ of a generally degrading existing roof, which is discussed further down.
3. The utter waste of time – This is the sort of repair that was never going to last because the roof is so poor, and has possibly made the life of any roofer harder should they come later to repair, or replace. In other words this will cost you more to put right, on top of the cost of the poor repair.

Temporary flat roof repair

These can be great for competent DIYers who aren’t fazed by heights. The trick with any flat roof repair is use the right repair material on the right type of roof. And not to use too much of it making it uneconomical, messy, or potentially more difficult to repair / replace professionally later on.

Suitable for cracks in felt, asphalt and lead.

Suited for repairs to felt,concrete and asphalt.

No flame, No Heat – Two super easy to do flat roof repairs that won’t burn your house down.

Now let’s look at some common roof types and possible repairs. Clicking on these links will ping you to that part of the page…

• Traditional felt roof repair – Either mineral finish, silver reflective coat, or grit on top.
• Asphalt roof repair – Grey in colour, looks like stone, but is bitumen based.
• Concrete roof repair – Looks err, like concrete unless coated.
• Fibreglass – Otherwise known as GRP.
• EPDM – Otherwise known as a rubber roof.
• Best paint on waterproofer – Emergency roof repair in a tin.

Felt roof repair

A felt roof could mean the now very old pour and roll type felts, or the newer and much better Torch-on roofing felts. Common problems with these roofs are…

1. Cracks, puncture holes (impact) or splits in the felt
2. Lifting or badly stuck joints in the felt
3. Leaking flashings into the wall of the house
4. Dips or hollows in the roof’s surface

Types of felt roof – These are all common types of flat felt roof, although a grit finish is hardly used these days. Mineral finishes to a felt can also be other colours like brown or purple.

 

 

Split in a felt roof – Even though this is a very old pour and roll built up felt roof, the splits on this roof are localised to a very small area that has been exposed to sunlight due to lack of grit. A paint on waterproofer alone could buy you a few weeks, or maybe a month or two. If you’re really lucky a year or two.

 

Lifting or badly stuck joints – This roof is full of dry joints, partly due to poor materials and workmanship, and partly through age again. One or two troublesome lifts like the one pictured could be temporarily repaired with a small amount of paint on waterproofer.

 

Leaking flashings – This can happen to any flat roof where it joins with a wall. As you can imagine, water blown against the wall will run behind it and into the property. As long as you don’t damage the roof while doing so, removal of old mortar and repointing with the correct mix usually solves this problem.

 

• DIY Temporary flat roof repair for felt roofs – This is often a paint on fix from a tin like the repair videos at the top of the page.
• Permanent flat roof repair – This can either be a patch of torched on roofing felt or a ‘layer on top’ of your existing felt if it will take it. There are also liquid systems for felt roofs, unfortunately the old felt you cover over may not be in good enough condition to take it, or for small domestic properties they often work out more expensive than a new roof.
• A waste of time – If your felt roof has the problem listed here, a tarpaulin over the top and a new flat roof will be your best bet.

DIY flat roof repair – This video is actually on how to repair a lead roof but can also be used on small damaged sections of felt roof too.

Warning

Make sure you are insured to work with naked flames. Always let any blowlamp warm up before using or tilting. Some blowlamps flare when tilted or the gas canister is full, be aware of this. Do not work with a blowlamp or a source of heat where it cannot be seen, or underneath flammable things like soffits, gutters etc. Be aware that even if insured, there can be a lot of ‘wriggle room’ if you haven’t taken adequate safety measures! Have a fire extinguisher on hand always, even if working with a simple heat gun as they can still start fires easily.

Asphalt roof repair

A good asphalt roof is very hard to beat, not only that but it lends itself to long lasting repairs in later life too. Bonus. Common problems with older asphalt roof are…

1. Blisters in the asphalt
2. Cracks, wrinkles or splits
3. Shrinkage or sagging of asphalt on vertical upstands or flashings

Cracks in asphalt roof – Cracks are often caused by age or thermal movement, often they cause no problems at all, especially if above any standing water. Unfortunately these can sometimes be opened up by standing water freezing and expanding over successive winters.

 

Bumps in asphalt roof – These blisters are caused by moisture trapped underneath the roof surface. Again as above, these generally cause no problem unless they burst and become hollow, or sit below lying water (ponding).

 

 

• DIY Temporary asphalt roof repair  – This is often a paint on fix from a tin, or heat applied flashing tape as seen in the video about repairing lead in the felt roof repair section.
• Permanent flat roof repair – This can either be a small patch of torched on roofing felt, or a ‘layer on top‘ of the asphalt roof and carried out relatively cheaply (a picture of this can be seen in the link on permanent flat roof repair). There are also several professional liquid systems that can repair asphalt roofs with good results, like Gacoroof or various 3M, and Kemper systems to name but three. The trouble is that the ones worth using are often only available to trade, or they can work out more expensive than a complete new roof on small domestic properties.
• A waste of time – In the unlikely event that your asphalt roof exhibits the problem listed via the link, a tarpaulin over the top, and a new flat roof will be your best bet.

The video for easy DIY lead roof repair using flashband can also be used of Asphalt to good effect, that video can be seen in the felt roof repair section above.

Warning – If you haven’t already please read this warning for working with heat.

 

Concrete roof repair

A concrete flat roof although undesirable these days because of insulation and condensation issues, was, and still is a long lasting roof. If you’ve ever heard the phrase ‘built like a brick outhouse’ you will know what that means, and this is its roof. The good news is that in later life they are very repairable for the most part. Common problems with concrete roofs are…

1. Cracks or splits in the concrete
2. Surface wear

Concrete roof leaks – A split or crack can obviously leak, but often they can be devilishly hard to find. Surface wear on the other hand may not cause a leak but can hinder repair, especially if the top surface breaks away or has the potential to puncture anything above it.

 

• DIY Temporary concrete roof repair  – This is a paint on fix from a tin, normally bitumen based like the video at the top of the page. A thin bitumen primer applied first and allowed to dry normally yields a better long term result. Often the top surface is cleaned of all loose chippings and moss and allowed to dry thoroughly before any waterproofer is applied.
• Permanent flat roof repair – If the crack is obvious, and the surface isn’t rough a small patch of torch-on roofing felt can work well, often a ‘layer on top‘ of the concrete roof is favourable, and a relatively low cost solution (see picture below).
• A waste of time – If the concrete was cast originally with too little cement they can become either rough or porous, emitting damp through and inside. If this is the case and a layer on top is not practical, a whole new roof including timber deck can be bonded or screwed onto the concrete ready for a new roof.

Concrete flat roof repair – Here a customer’s concrete roof is shared with a neighbour’s, their side wasn’t leaking so a layer on top was applied to the customer’s side alone, after a thorough clean and prime to ensure good adhesion.

 

Epdm rubber roof repair

Rubber roofs are very easy to repair either professionally, or in an emergency DIY fashion. In the event of a small tear, cut, or hole in a rubber roof (normally from roof tile or slate impact), the secret is to use an approved rubber roof product rather than any other sort of roof repair material. Common problems with EPDM are…

1. Shrinkage from lack of mechanical fixings near edges or upstands
2. Poor roof detailing on flashings or joints
3. Holes or cuts from sharp edge impact or vandalism

EPDM Rubber roof – These are common examples of a poorly installed rubber roofs. Wrinkles, contraction of rubber membrane, ponding and poor detail work were present, as well as being very ugly to look at. Impact holes or cuts are very repairable by competent DIY’ers though.

 

• DIY rubber roof repair – If the hole or tear is small and you want the fastest fix possible to buy you some time, a tube of lap sealant applied sparingly to the affected area should do the trick. Remember don’t go berserk as it will have to be scraped off later on for a more professional repair, you don’t want to make it harder or more expensive.
• Permanent EPDM repair – This usually consists of a relatively simple patch. Firstly buy a self adhesive flashing, most often they are offered in two types. A highly malleable rubber flashing tape often with the word ‘form’ in it, because it forms into tricky shapes. These are mostly used for flashing round corners or bends. Alternatively you can get a thicker self adhesive flashing which isn’t as malleable but stronger. Whichever you buy make sure you buy a self adhesive rubber tape that will provide minimum 75mm (3″) of coverage either side of the hole to be repaired. The roof will have to be thoroughly cleaned in the area of the repair first and allowed to dry. Next use an EPDM primer applied with a scrubber pad, when touch dry, firmly adhere the patch from the centre outwards with a hard roller. Lap sealant can also be applied around the edges of the repair as an extra precaution, especially if it’s lying in water.
• A waste of time – In the case of a rubber roof this nearly always means a very poorly laid rubber roof. Unfortunately the apparent ease of laying a roof like this has encouraged ambitious DIY’ers to overreach themselves, and a strong cowboy sector to have a go too. Common faults are poor joints, no mechanical fixings near edges, contraction stretching with age, and poor flashing details resulting in leaks. Often unfortunately, small repairs are also cheaper for a professional rubber roof installer to do rather than buying all the components yourself.

Fibreglass roof repair (GRP)

Fibreglass roofs are also very easy to repair if you have to. The key when working with GRP is to use repair products that are meant for use with fibreglass so no further difficulty or damage is done. Common problems with these roofs are…

1. Flashings leaking near abutting walls.
2. Cracks or splits near upstands, trims or board joints.

Leaking fibreglass roof – This roof holds water (ponds) quite a bit as you can see. This is often mistaken for the cause of a fibreglass roof leak. If you look at the photo you can see the mortar missing from the chase, and where it’s not missing it’s cracked and loose.

 

• DIY fibreglass repair – The first place to check with a fibreglass roof is the flashings into any abutting wall. I find that the GRP wall flashings often used can cause problems where a sand and cement mortar is used, or sealant is poorly applied, i.e. into a dusty brickwork chase. Find a link for roof flashings at the bottom of the page if you need it. Next in the list is a crack caused by fatigue or expansion and contraction. Two common causes for this are lack of reinforcing on joints and trims, or not allowing for expansion correctly in the planning stage. A very good temporary repair for a crack or impact hole is the use of a small amount of Acrylic sealant just a few millimeters either side of the affected area, this can also be reinforced with a small amount of plasterers scrim if wide or deemed necessary. The roof must be cleaned of dirt or algae in that area first with warm soapy water for the repair to take. Acrylic sealant will scrape off and clean with acetone for a permanent repair later on.
• Permanent fibreglass roof repair – This involves a thorough cleaning and sanding of the affected area first with acetone and 60 grade grit paper for a minimum of 75mm (3″) around the repair. A 600g chopped strand mat (CSM) can then be laminated into place (resin onto roof area to be repaired, c.s.m. cut to size, resin on top, all bubbles removed). Then after curing, a matching topcoat colour is applied.
• Waste of time – In the case of fibreglass this normally means something has gone horribly wrong with the lamination, this could mean delamination, resin starvation, contamination etc. Get a G.R.P. roofer in to take a closer look.

Temporary repairs to a bitumen based or lead roof are often a paint on waterproofing solution straight out of a tin for most DIY purposes, and they can work very well. What I would say is if you can’t keep the repairs small and localised you may not want to try, the bigger you go with a paint on system the more difficult a potential professional repair may be later on, if possible. Often paint on waterproofers will be a bitumen based paint or acrylic in nature. Lots of this daubed all over a roof may make it impossible for a hot flame repair, patch, a layer on top, or a liquid roof repair later, should the rest of the roof be professionally repairable.

Best waterproof paint – These are the three main contenders for DIY. Acrylic based, bitumen based and a rubber type paint. The Acrylic based is most favoured by professionals for the reasons listed below.
The best paint on waterproofer for me, hands down, is an Acrylic based waterproofing paint like Cromapol. You will often find a tub of this in any professional roofers kitbag for small repairs because it doesn’t make a damned mess, looks pretty, works, and keeps well. In fact it’s so popular it’s made by many companies with similar sounding names like, Evercryl, Acrypol, Roofacryl. On the downside it isn’t dirt cheap, but it can be bought in a mastic gun tube for very small tasks.

An acrylic waterproof coating can be used in the wet as stated on the side of the tin but, not on top of dirt, grease, moss or algae. And best results are obtained in the dry naturally.

Can I paint my whole flat roof with a waterproof sealer ? (Felt, Asphalt, Concrete)

The answer here is yes and no. Lets imagine you have a small flat roof, 2 metres by 2 metres (6ft x 6ft) for example, and upon inspection there are numerous splits or holes. If you go for a cheap option like a large tub of bitumen type sealer you could apply it with a soft bristled yard brush, or a giant paint brush and coat the whole roof with a nice thick coat. The trouble here is longevity. Assuming you manage to get a nice even coat, bitumen based sealants eventually dry out and crack, and the larger the roof is and the poorer the initial condition, the less reliable it becomes.

Scenario two is that we spend some money on a good quality sealant like an Acrylic paint, polymer, or rubber based system. Now we’re in the land of spending a not insignificant amount of money for what is in effect a roof repair. Yes, this will last longer than a paint on bitumen repair, if you can coat it at a nice even thickness that is, and not make any mistakes. Even for professionals one of the problems with any paint on system has always been guaranteeing consistency of application, and this leads us nicely into the next problem, Price v Return. Often the cost of purchasing enough sealant means it will still not pay back for the time the repair will last. So unless it’s a very small surface area we could well be back in the waste of time category.

Permanent flat roof repair (Felt, asphalt, concrete)

What is a permanent flat roof repair and what do you mean by it? Well, there is no such thing as permanent, or I would be out of a job. But a repair that adds significant life to an existing roof, and I’m thinking 10 years plus here, seems like a fairly permanent repair to me. There are three possible candidates here for me.

Asphalt roof repair – This is a prime example of a layer of torch-on felt lamped onto an old asphalt roof. All cracks and blisters were dealt with first before one layer of thick high performance felt, finish with solar reflective paint for future repair if necessary. A permanent value for money repair. This can also work for felt and concrete.

 

1. Professional patch repair – This is usually a heat applied patch similar to a polyester Torch on felt, or a good quality liquid system well applied.
2. A layer on top – There are times when a felt roof or an asphalt roof nears the end of it’s life expectancy, in other words too many small niggles to make a patch repair viable (as seen above). If the roof and structure beneath are in reasonable condition it may take a new Torch-on felt roof laid straight on top. This is usually done by applying a single layer of thick polyester felt onto a clean, grit and dirt free roof with a very large blowlamp. To finish, new edge details and flashings go on. Although frowned upon by some in the roofing trade, I have personally seen these relatively low cost procedures last in excess of 20 years. Often. Although I curse when stripping them off in later years for a new roof when I didn’t realise they were there.
3. A professional liquid based roofing system – The big problem here being that the cost is often more than the price of a new flat roof on domestic houses. I have looked into these several times, and I cannot sensibly make them pay. Why would you spend more or the same money on essentially a repair, rather than replace with a new roof and a guarantee? (On very large flats or commercial buildings it can pay, because of the economies of scale)

When repairing a flat roof is a waste of time

Let’s deal with this one as best as we can. There’s a lot of scope here because there are just so many roofing systems and possibilities to choose from, so I’m going to choose the most common. The very first thing to look at is the structure itself, and the first few seconds when a roofer first pokes their head over the top of your flat roof he will be looking at the roof covering, and the condition of t